Excel on Mac using commas to represent decimals

Go to:

System Preferences -> International -> Input Menu

Check off

Show input menu in menu bar

Look up in the menu bar, and make sure the country selected is correct for you ( Assuming US ). If it's set for a different country, you may see this issue. I believe if your input is set to German, for example, it will exhibit what you describe seeing in Excel.

EDIT: details on solution from @sixtyfootersdude:

This is what my international panel looks like. You can see that there is a little warning there. Here is what resolved the problem:

I changed the Region to US I closed the panel I opened excel (everything is now using periods) I quit excel I reopened the international panel I changed the region back to Canada Apparently this solves the problem.